1.0 Field of the Invention
The present invention relates generally to headsets for sound reproduction and, more particularly, to a headset which reproduces sound in a manner which enhances the spatial or three-dimensional quality of the sound as perceived by the listener.
2.0 Related Art
Headsets have been in widespread use for some time for the purpose of providing personal musical entertainment. More recently, headsets have been used as an integral part of a variety of interactive video systems. Headsets include a pair of headphones, with one cupped over each ear of the listener, and a headband which separates and is attached to the headphones. Each headphone includes at least one acoustic transducer for the purpose of reproducing sound.
Human hearing is spatial and 3-dimensional in nature, i.e., a listener with normal hearing his aware of the spatial location of objects which produce sounds in his environment. Natural spatial hearing, which is also referred to as binaural hearing, permits a person to identify the locations of a variety of sound sources, such as musical instruments or voices, occurring simultaneously and to discern the location and direction of movement of moving objects such as motor vehicles.
As sound reproduction systems have advanced from early monaural systems to stereophonic and later quadrophonic systems, those skilled in the acoustic arts have applied a variety of sound enhancement technologies to headphones to improve the listening experience. For instance, U.S. Pat. No. 4,821,323 issued to Papiernick discloses a stereo headphone having a pair of conventional speakers as well as a pair of secondary vibrational audio output discs adapted to rest against the temple of the user, to transmit sound waves through the bones and tissues of the user's head, as well as through the use's ears, to simulate a concert hall listening environment.
A common goal of many headphones has been the elimination of "in-the-head" sound of earlier headphones which produced sound lacking binaural cues and the production of sound having enhanced spatial, or 3-dimensional quality as perceived by the listener. For instance, U.S. Pat. No. 5,175,768 issued to Daniels discloses electrical circuitry and the associated methodology to provide effective simulated acoustic cross coupling in stereo headphones, noting that the lack of acoustic coupling (i.e., sound shared by both ears) results in a sound field lacking depth. Instead the sound is compressed and wedged into the central upper portion of the head. Daniels further notes that cross coupled signals have been demonstrated to aid in headphone listening, with most listeners reporting a marked sense of spatial expansion to the resultant sound field. U.S. Pat. No. 3,939,310 issued to Hodges discloses a stereophonic headset having an enclosed ear-to-ear acoustical passageway which is provided so that each of the listener's ears hear the output of both headset stereophonic speakers.
The production of multi-dimensional sound, typically referred to as 3-D or 3-dimensional sound, has become particularly desirable due to the work being done in the field entitled "Virtual Reality" which includes both 3-dimensional visual displays as well as 3-dimensional sound. For instance, with the advent of home computers and interactive visual communication systems using home television sets as a video display means, it has become increasingly desirable to generate 3-dimensional sound or sounds associated with an object or objects appearing on the television screen and further to permit the listener and viewer to make interactive decisions with the images displayed on the screen.
It is known in the acoustic arts that sounds which have binaural location cues permit a listener to locate the source of the sound in 3-dimensional space. It is further understood that these cues are created primarily by the intensity and phase (time of arrival) differences between the sound at the two ears of the listener, as well as the spectral changes of sound resulting from the complex shape of the pinnae, or outer ears. This spectral modification may also be affected by the head and torso of the listener. It is further recognized that the left-right directional sense of sound is perceived by the interaural time difference and intensity difference when the sound waves reach the head, while the primary physical cause of up-down and forward-backward directional perception is sound wave distortion, or spectral modification, caused by the pinnae. Furthermore, it is understood that the earphones of headsets disturb the conch resonance of the pinnae.
An early method of simulating the production of 3-dimensional sound recording utilized a dummy head ("kunstkoph"). With this method, two recording microphones are placed within the ears of an anthropometric mannequin, with the ears being formed to replicate human ears. Although this method may simulate the reproduction of 3-dimensional or spatial sound, any simulation of spatial sound which may occur is based upon the mannequin ears and not the ears of the listener. Accordingly, the intended simulation may be adversely affected due to the difference in the material used to construct the mannequin ears, as compared to the flesh and cartilage of the human ear, and due to any difference in the particular shape of the mannequin ears as compared to the ears of the listener. Additionally, although the original spatial location of the sound may be captured, it may not be edited or modified. Accordingly, this earlier mechanical means of binaural processing is not useful in a video game for example, where the sound must be interactively repositioned during game play.
U.S. Pat. No. 4,817,149 issued to Myers discloses a later development comprising a binaural signal processing circuit and method which is capable of processing a signal so that a localization position of the sound can be selectively moved. Elevation and front/back cues are established utilizing direction-dependent filtering while horizontal (azimuthal) localization is achieved by interaural time differences.
Recent developments in binaural processing use a digital signal processor (DSP) to mathematically emulate a dummy head process in real time but with positionable sound location. Typically, the combined effect of the head, ear and pinnae are represented by a left-right pair of head-related transfer functions (HRTFs) corresponding to spherical directions around the listener, usually described angularly as degrees of azimuth and elevation relative to the listener's head. The HRTFs may arise from laboratory measurements or may be derived by means known to those skilled in the art. Right and left ear binaural signals may then be produced by applying a mathematical process known as convolution wherein the digitized original sound is convolved in real time with the right and left HRTFs corresponding to the desired spatial location. The sound reproduced from these binaural signals, when heard, appear to originate from a desired location. A sound may be repositioned by changing the HRTFs to those for the desired new location.
Although DSP-based binaural systems are known to be effective, they are also known to be costly because the required real time convolution processing of a sound imposes a substantial computing burden. U.S. Pat. No. 5,521,981 issued to Gehring discloses alternative apparatus which eliminates the need for a DSP and real time binaural convolution processing and provides means to achieve real time, responsive binaural sound positioning. The burdensome processing task of binaural convolution required for spatial sound is performed in advance by a preprocessing means.
U.S. Pat. No. 5,438,623 issued to Begault discloses a multi-channel spatialization system for audio signals which is illustrative of another system utilizing HRTFs for producing 3-dimensional audio signals. The stated objectives of the disclosed apparatus and associated method include, but are not limited to: producing 3-dimensional audio signals which appear to come from separate and discrete positions from about the head of a listener; and to reprogrammably distribute simultaneous incoming audio signals at different locations about the head of a listener wearing headphones. Begault indicates that the stated objectives are achieved by generating synthetic HRTFs for imposing reprogrammable spatial cues to a plurality of audio input signals received simultaneously by the use of interchangeable programmable read-only memories (PROMs) which store both head related transfer function impulse response data and source positional information for a plurality of desired virtual source locations. The analog inputs of the audio signals are filtered and converted to digital signals from which synthetic head related transfer functions are generated in the form of linear phase finite impulse response filters. The outputs of the impulse response filters arc subsequently reconverted to analog signals, filtered, mixed and fed to a pair of headphones. Another aspect of the disclosed invention is to employ a simplified method for generating synthetic HRTFs so as to minimize the quantity of data necessary for HRTF generation.
Based on the foregoing, it may be seen that there is a continuing need in the acoustic arts to provide headphones which deliver 3-dimensional, or spatial sound to the ears of the listener in a simple and economical manner.